POTASSIUM and POTATOES

When it comes to growing potatoes, potassium is the number one nutrient. Potatoes use more potassium than any other plant nutrient including nitrogen.

“Potatoes have a higher demand for potassium than any other vegetable crop,” says Don Horneck, extension agronomist at Oregon State University at Hermiston. This comes from a researcher who has worked with a variety of vegetable crops including onions, peas, lima beans and spinach.

His research has shown potato yields can increase between 8 and 10 tons per acre when potassium is applied at rates of 600 to 700 pounds per acre compared to a check where no potassium is applied. However, research done in Washington State in the late 1990s showed little yield or quality response to potassium fertilization.

The difference may lie in soil potassium test levels. When soil tests are in the moderate range, 150 parts-per-million or greater, growers find it is easier to manage fertilizer applications. “At higher soil test rates, it’s easier to get the system to work for you,” Horneck says.

But when soil tests are low, say around 50 ppm, and high potassium demand crops are grown, growers need to pay closer to attention to application rates and timing.

A traditional crop rotation in the Hermiston includes three years of alfalfa followed by one year of potatoes. Two high potassium demanding crops combined with sandy soils can drop potassium soil levels to that low range.

To ensure potassium is available when plants need it, many growers in the Hermiston area apply at lay by. During the two- to three- week period when a potato plant’s demand for potassium is greatest, potato vines in northeastern Oregon can use as much as 15 pounds per acre per day. In neighboring Idaho, with its different soils, demand is about half at 7 to 8 pounds per acre per day.

Although potato growers in the Pacific Northwest apply up to 70 percent of seasonal nitrogen fertilizer through sprinkler irrigation systems, the practice is not common for potassium fertilizer. Potassium must be solubilized before it can be run through an irrigation system, and that is expensive. Nearly all potassium is applied in the granular form.

When it comes to selecting a potassium fertilizer, the potato plant doesn’t care what the potassium source is. However, there are situations when choosing potassium sulfate or SOP over potassium chloride makes sense.

Growers in regions where soil potassium tests are low and large amounts of potassium fertilizer are required to meet the plant’s nutrient needs will benefit from a split application. The general recommendation is that if more than 300 pounds of potassium per acre is required, growers should apply half as potassium sulfate and half as potassium chloride.

Bryan Hopkins, a soil scientist at Brigham Young University at Provo, Utah, has also farmed. Over the years, he has had fields where salts and chloride levels were high, and he chose to use a combination of potassium sulfate and potassium chloride. When it’s been economically advantageous, he has increased his use of potassium sulfate and reduced the amount of elemental sulfur.

Growers who choose to band apply fertilizer can apply higher rates of potassium sulfate because of the lower salt content. Potassium sulfate is less mobile in the soil, which is also an advantage when banding fertilizer.

Areas where sodium (salt) or chloride levels are already high in soil or irrigation water may also benefit from potassium sulfate. Excess salts in the soil can result in water being held so tightly the plants can’t take it up. Potassium chloride is extremely mobile in the soil and may not be in the root zone when plants need it.

Chloride, like elemental sulfur, is an essential nutrient for potato growth. Potato plants require 60 pounds per acre, while chloride uptake has varied from 10 pounds to over 100 pounds. But just like your mother tells you, too much of anything is not good.

POTATO farmers get the latest on the fertility needs of their crop at a recent field day.